In recent years, optical communication systems of further longer distance and further larger capacity have been required for multimedia networks, along with the construction of optical networks using them. A WDM system is one of the systems for realizing large capacity, for which considerable research and development have been carried out
Furthermore, in WDM optical communication systems, it is required to increase a transmission speed (bit rate) per one wavelength according to the development of high speed electronic circuits in optical sender and receiver, to increase transmission capacity. This requires upgradability capable of responding to a higher transmission speed, and the use in a state where optical signals of a plurality of bit rates, for example 40 Gb/s, 10 Gb/s, 2.5 Gb/s and the like, are intermixed.
An example of WDM optical communication system that is currently being developed is a system in which the highest transmission speed is approximately 10 Gb/s, and the wavelength spacing (wavelength arrangement spacing) is 50 GHz or the like. In an optical transmission terminal apparatus, an optical reception terminal apparatus and an optical add drop apparatus, which constitute such a WDM optical communication system, processes of multiplexing optical signals with respective wavelengths (channels), and demultiplexing them for each wavelength are performed. At the time of such multiplexing and demultiplexing of wavelengths, an optical filter is typically used, so that flatness in a filter band of each channel becomes important. That is, if the flatness is insufficient, then a part of signal light component is cut off, thus producing waveform distortion. Therefore, the flatness over a wide band is required as the transmission speed gets higher. Furthermore, if a large number of optical add drop apparatuses and the like are inserted in the transmission path, since the number of filters through which the signal light passes are increased, the more strict flatness is required.
In order to satisfy the flatness in a filter band, in the case of transmitting an optical signal of 10 Gb/s at 50 GHz spacing, for example, a technique is known in which an optical filter called an interleaver is combined with an optical filter that demultiplexes or multiplexes optical signals at 100 GHz spacing or 200 GHz spacing utilizing an AWG, a film filter or the like. In this conventional technique, in the case of demultiplexing WDM signal light, for example, the WDM signal light is demultiplexed into two signal groups using the interleaver, and further, light of each optical group is demultiplexed into optical signals with each wavelength by the optical filter utilizing the AWG, the film filter or the like. As shown in FIG. 17, the above-described interleaver is a known optical filter that has a function for demultiplexing input signals (upper part of the figure) at 50 GHz spacing into a signal group A (solid lines in the figure) at 100 GHz spacing, and a signal group B (dotted lines in the figure) at 100 GHz spacing, which is shifted by 50 GHz from the signal group A. Here, the middle and lower parts of FIG. 17 show output signals and demultiplexing characteristic of the interleaver, corresponding to the signal group A.
In trying to demultiplex optical signals at 50 GHz spacing using only an optical filter using an AWG, a film filter or the like, it is difficult to ensure flatness in a filter band. However, by separating into signals at 100 GHz spacing using the interleaver as described above, filter bandwidth of the AWG, the film filter or the like is extended, so that it is easy to obtain band flatness.
Here, there has been shown the case where the WDM signal light is demultiplexed into each wavelength. However, in the case of multiplexing optical signals with different wavelengths at 50 GHz spacing, it is also possible to multiplex respective optical signals by reversing an input and output relationship of optical signals at the time of multiplexing as described above.
Incidentally, in the conventional WDM optical communication system as described above, in order to accommodate as many optical signals as possible in a predetermined wavelength band, there is a tendency to attempt to narrow the wavelength spacing of each optical signal. However, since a required bandwidth for optical signals with each wavelength exists according to the transmission speed, there is a limit when trying to narrow the wavelength spacing. That is, if the transmission speed of an optical signal gets higher, the required bandwidth as described above is widened. Therefore, if the wavelength spacing of each optical signal is made too narrow, such bandwidth cannot be ensured, resulting in a problem in that the transmission cannot be performed.
For example, if it is attempted to transmit an optical signal with a transmission speed of 40 Gb/s at 50 GHz spacing, crosstalk with neighboring wavelengths is predicted, so the optical transmission at such wavelength spacing is considered to be difficult. Therefore, in a 40 Gb/s WDM optical communication system, investigations with the wavelength spacing of 100 GHz have been made.
Furthermore, as described above, WDM optical communication systems are required to be used in the state where optical signals of various bit rates are intermixed, and to have upgradability to higher speed bit rates. In order to use the conventional WDM optical communication system in the state where optical signals of various bit rates are intermixed, since it is necessary to select the wavelength spacing from the beginning (designing stage) so as to be adapted to an optical signal of a maximum bit rate supported by the system, there is a problem in that accommodation efficiency of optical signals of lower bit rates becomes poor. Moreover, in a system wherein the wavelength spacing is optimized corresponding to one existing bit rate, if the bit rate is made higher, there occurs necessity to increase the bit rates of all channels, and to exchange or reduce optical multiplexers and demultiplexers, and the like. Therefore, there is another problem in that it is difficult to respond to higher bit rates flexibly,
The present invention has been accomplished in view of the above-described problems, with an object of providing a WDM optical communication system that can efficiently arrange wavelengths of optical signals of a plurality of bit rates at different wavelength spacing, and can respond to the upgrade to higher bit rates flexibly.